We were caught in a rain while we were on a picnic.

英訳に間違いが有れば正してください。

私達はピクニックの途中で雨にあった。
We were caught in a rain while we were on a picnic.

答え rainをshowerにする

We were caught in a shower while we were on a picnic.

be caught in a rainという表現はない

rain
形容されなければ不可算名詞

It looks like rain.
雨模様だ

in a rain storm
in a rain shower
などの使い方は有り

I like to walk in a soft spring rain.
静かな春雨の中を歩くのが好きだ。